The present invention relates to a semiconductor laser and, more particularly, to a semiconductor laser of effective index type having the stripe ridge structure.
FIG. 2 is a schematic sectional view showing an example of the semiconductor laser of effective index type having the stripe ridge structure. This semiconductor laser is based on AlGaInP and is designed to make recording on a DVD (Digital Versatile Disc).
The semiconductor laser shown in FIG. 2 is comprised of a substrate 101 of n-type GaAs, a lower cladding layer 102 of n-type AlGaInP, an active layer 103 of GaInP, an upper cladding layer 104 of p-type AlGaInP, and a contact layer 105 of p-type GaAs, which are sequentially arranged upward. The contact layer 105 and the upper part of the upper cladding layer 104 form the stripe ridge structure a. The upper cladding layer 104 (spreading over the foot and slope of the ridge a) is covered by the buried film b, which is comprised of a low refractive index layer 106 of AlInP and an anti-oxidizing layer 107 of GaAs. The contact layer 105 is covered by and connected to an upper electrode 108, and the substrate 101 is provided with a lower electrode 109. The buried layer b, which is comprised of a low refractive index layer 106 and an anti-oxidizing layer 107, reduces the internal loss of the laser beam generated, which is disclosed in the Japanese Patent Laid-open No. 2002-198614 (paragraphs 3, 9, and 10) (hereinafter referred to as Patent Document 1).
Any semiconductor laser to be used as an optical pickup for recording is required to have a high output to increase the recording rate. FIG. 3 shows the L-I curve (light output versus current characteristics) of the aforesaid semiconductor laser based on AlGaInP. It is noted from FIG. 3 that the semiconductor laser usually increases in light output almost linearly in proportion to current above the threshold value. Unfortunately, the increased current causes the transverse mode of the laser to shift from the zeroth-order mode (fundamental mode) to the first-order or higher-order mode. As the result, the light output does not increase linearly any longer. This phenomenon is called “kink” (particularly higher-order mode kink). The light output (indicated by Lk in FIG. 3) at which this phenomenon occurs is referred to as the kink level. This kink level Lk often determines the maximum output of the semiconductor laser.
The condition for preventing the higher-order mode kink is represented generally by the equation (1) below.
                    W        ≦                              λ            0                                2            ⁢                                                            n                  1                  2                                -                                  n                  2                  2                                                                                        (        1        )            where,    n1: effective refractive index of ridge a    n2: effective refractive index of buried film b    λ0: wavelength of laser beam    W: width of ridge    as shown in FIG. 2.
The equation (1) above suggests a possible way of preventing the higher-order mode kink by reducing the value of W (the width of the ridge) or by reducing the value of n1−n2 (the difference between the two effective refractive indexes). To achieve this embodiment, there has been proposed an idea of modifying the semiconductor laser shown in FIG. 2 by replacing the low refractive index layer 106 of AlInP with a semiconductor layer of AlGaAs having a refractive index of 3.4 to 3.55. This modification causes the value of n1−n2 to decrease to 0.001 to 0.005, thereby making it easy to attain the fundamental mode. (See Patent Document No. 1.)
In addition to the above-mentioned two methods, there is a third possible way of preventing the laser transverse mode from shifting from the zeroth-order mode to the higher-mode order. This embodiment is achieved by increasing the coefficient for absorption of higher-order mode in the buried film. A practical structure for this purpose is shown in FIG. 2, in which the low refractive index layer (AlInP) 106 is made thin so that the anti-oxidizing layer (GaAs) 107, which functions also a laser absorbing layer, comes close to the active layer 103.